Some applications require a load to be supplied with modulated power. For example, a light source like a light emitting diode (LED) or a laser diode may be supplied with modulated power, which causes the light source to emit a modulated light signal.
One example application for such modulated power supplies supplying a light source are Time-of-Flight (ToF) sensors. Such sensors emit a modulated light signal which is reflected by a nearby object towards an image sensor comprising a matrix of photon mixing devices (PMDs) that resolve the object in real time. Each PMD in the matrix measures a distance to a point on the nearby object by determining a phase shift between the emitted light signal and the reflected light signal. The information gathered across the matrix from all PMDs may be used for applications like automobile crash detection systems, 3-dimensional imaging, video game interfaces or robotics, but are not limited thereto.
For modern Time-of-Flight sensors, modulation frequencies in the MHz region are used for generating the modulated light signals. Therefore, corresponding modulated power supplies are needed which supply one or more light sources used with modulated power.
In one approach, a direct current/direct current (DC/DC) converter is used to provide a suitable input voltage (for example based on a battery voltage) to a desired operating voltage. A modulation switch is provided to enable or disable current flow through a light source like a light emitting diode (LED). A blocking capacitor is furthermore provided for decoupling during switching. Additionally, a resistor is used for current limiting. While such a conventional approach has a simple structure, it requires a comparatively large area for a blocking capacitor, and the power efficiency is comparatively low due to the current limiting resistor. Moreover, the turn-off time of the light emitter may be comparatively high.
In another approach, instead of providing a modulation switch coupled directly to the light emitting diode, a modulation switch is coupled to a node between a DC/DC converter and a light source with one terminal and to ground with another terminal. When this switch is turned on, it shunts the current to ground thus turning the light emitting diode off. This approach may reduce turn-off time by reverse biasing of the light source. On the other hand, comparatively high power losses may result.
Another approach combines the approaches mentioned above, by providing for example a first modulation switch between an anode of a light emitting diode used as light source and ground and a second modulation switch between the cathode of the light emitting diode and ground, the anode being coupled with a DC/DC converter.
It may be desirable to provide further possibilities for modulated power supplies, in particular for use for supplying light sources, for example, in Time-of-Flight sensors.